Numerous examples exist of types of rock or ground stabilisation bolts having the form of a rigid bar. The rigid bar generally has an elongated shank for insertion in a borehole drilled from an excavation into surrounding rock, which is to be contained or stabilised. The installed bar acts as a rock bolt, which together with a plate and nut provided at one end of the bar serve to reduce the risk of collapse of the rock forming the roof or walls or uplift of the floor of the excavation.
The borehole is usually drilled to a depth so that one end of the rigid bar and at least a portion of the length of the bar adjacent to this one end is secured to relatively stable rock by a fast setting resin mix, other grout formulation or mechanical anchor device.
Such rigid bars are often of limited use where a borehole must be drilled deep into the roof of the excavation before relatively stable strata is located or where thicker zones are to be reinforced. The rigid bars are relatively inflexible, and thus a bar of greater length than the height of the mine or tunnel or any other type of excavation cannot be installed without being plastically deformed and then straightened again before being inserted into the borehole. Rigid bars of a particular diameter also have a relatively limited load carrying capacity and therefore a relatively large number of rigid bars must be used over any given area to achieve the required support or reinforcing action.
A cable form of rock bolt is shown in German Patent Application DE3435117A. The cable form of rock bolt disclosed therein has a rigid end or sleeve portion formed at the end of the cable part of the bolt to enable a plate and nut to be fitted to the bolt. The rigid end is usually preformed on the cable by casting or swaging for example, and therefore the cable bolt is provided in a predetermined length. Accordingly, a cable bolt must be ordered and supplied to the excavation site, depending on the borehole depth. This is often not practical, where the depth of boreholes needs to be varied from area to area.
Another cable form of rock bolt is disclosed in U.K. Patent Specification No. GB2084630A. The cable disclosed therein has an anchored swivel at one end of the cable which is inserted into the borehole in order to secure the bolt. At the other end of the bolt there is provided a portion of rigid bar onto which a plate and nut can be fitted. In manufacturing the rock bolt GB2084630A difficulty is encountered in attaching a rigid bar to the cable and also relatively higher costs are involved in its manufacture. Problems similar to that of DE3435117A with regard to varying borehole depth equally apply in respect of the bolt disclosed in GB2084630A.
A further problem encountered with rigid bar bolts as noted above is their limited load carrying capacity per unit bolt diameter. When the rigid bar bolt is in situ, the load of the rock forming the immediate roof of the excavation which is to be supported is transferred to the rigid bar or known cable form via a plate by means of the threaded area between the nut and rigid end of the known bolts.
Devices of this general type which are inserted into drillholes and bonded to the rock are subject to possible axial forces and shear forces, the latter occurring as a result of at least partial sideways movement of certain rock zones. Thus, to prevent premature yielding of the device when rigid bars are used, there is a tendency to use bars of greater diameter. However, this necessitates use of a heavier and more expensive bar and requires a larger diameter borehole to be drilled into the rock. It would be seen of advantage to keep the diameter of the exposed end of the bolt small because small holes are more suited for maximum drilling speed and to form a small annular zone between the borehole and the bolt for efficient resin mixing and maximum bond strength development. It would be an advantage to provide a cable rock bolt which is able to carry larger loads than that of known rigid bars of the same diameter so that borehole diameters and time of drilling and installation can be kept to a minimum.
It has also been found to be sometimes difficult to agitate resins in the borehole to ensure correct mixing of constituents due to the substantially cylindrical nature of some prior art bars.